I. Field of the Invention
The present invention relates to hemming machines and, more particularly, to a sheet metal hemming machine utilizing a vertically movable nest supporting the sheet metal part to be hemmed to press against stationary dies which are moved into and out of an operating position.
II. Description of the Prior Art
Typically sheet metal hemming devices utilize hydraulic cylinders for imparting vertical movement to the impacting punch or nest. The vertical movement provided by the cylinders is often erratic which slows down the operation of the hemming process and otherwise causes inaccuracies. This is particularly critical where multiple actuating hydraulic cylinders are employed, even if in final, this compliance allows a perfect balancing of the hemming effort on each edge of the part.
A major disadvantage of the previously known use of hydraulic cylinders for hemming machines is the inevitable leakage of the hydraulic fluid and the resulting mess created by such leakage.
There have, however, been previously known hemming machines which utilize electric servo-motors in order to vertically displace the nest to perform the hemming operation. These previously known electric motor actuated hemming machines, however, have required a complete synchronization between the rotation of the actuating shafts in order to achieve the vertical displacement of the nest. In practice, however, such absolute synchronization between the rotatable shafts cannot be achieved in every situation, and does not allow a perfect balancing of the hemming effort applied on each edge of the part, in both pre-hemming and hemming positions. Furthermore, any lack of synchronization between the electric motor actuated shafts may result in seizure and even destruction of the hemming machine.